1. Field of the Invention
This invention relates to the removal of sulfur oxides from flue gas streams by injecting a soda-type dry sorbent such as trona or nahcolite into the gas stream and collecting the sorbent in a bag filter.
2. Description of the Prior Art
Dry sorbent injection is receiving increased attention as a method for removing sulfur oxides from the stack gases of coal-fired utility and industrial plants, especially those using low sulfur coals. Flue gas desulfurization (FGD) systems in commercial use are generally based on wet scrubbing or spray drying processes. Evaluations of dry sorbent injection, however, have revealed its advantages of operational simplicity and reliability, attractive equipment and labor costs, and lower water consumption, over the present conventional FGD systems.
In dry sorbent injection, a suitable sorbent is injected as a dry powder into a SO.sub.2 -containing flue gas stream and is then collected, along with fly ash in the hot flue gas stream, in a fabric filter baghouse.
Sulfur dioxide in the hot flue gas stream reacts with the injected sorbent, while the latter is fluidized in the ductwork and also when the flue gas passes through the baghouse filter cake. The baghouse filter cake, a mixture of fly ash and sorbent-SO.sub.2 reaction product, is periodically removed for disposal.
Sodium compounds are currently favored as dry injection sorbent candidates, with nahcolite (crude sodium bicarbonate), trona (crude sodium sesquicarbonate), and absorptive soda ash (commercial sodium carbonate) being mentioned as suitable soda-type sorbents.
The performance of these soda-type sorbents is generally measured by two criteria:
(i) sulfur dioxide removal efficiency--the requirement that a specified percentage, e.g., 70% or 90%, of the SO.sub.2 content in the SO.sub.2 -containing flue gas stream be removed by FGD treatment with sorbent; SO.sub.2 removal efficiency is readily calculated as ##EQU1##
(ii) sodium utilization--the percentage of available sodium in the dry sorbent which is reacted with sulfur dioxide during the FGD process; "sodium utilization," sometimes referred to as "sodium efficiency," may be calculated as ##EQU2##
A third parameter related to these two performance criteria is the "normalized stoichiometric ratio" (NSR), a measure of the overall reaction stoichiometry during the FGD treatment process. Normalized stoichiometric ratio may be calculated as ##EQU3##
A NSR=1 for a flue gas desulfurization treatment indicates that for every one mole of sulfur dioxide present in the flue gas stream being treated, there is also present one mole of available sodium (Na.sub.2) in the sorbent.
A desirable sorbent is one in which the percentage of SO.sub.2 removed from the flue gas stream is maximized, e.g., SO.sub.2 removal efficiency =90+%, while the sorbent is fully utilized during the desulfurization treatment process, i.e., substantially all available sodium is reacted.
In recent full-scale dry-injection FGD tests involving nahcolite, trona and soda ash as the dry sorbents, nahcolite was reported to be superior to trona in SO.sub.2 removal at comparable NSR's, and soda ash was found to be ineffective in removing SO.sub.2. With nahcolite as the preferred dry sorbent, a steadystate SO.sub.2 removal efficiency of 81% was achieved with a NSR=1.0 (corresponding to a sodium utilization of 81%); see L. J. Muzio et al., "22MW Coal-Fired Demonstration of Dry SO.sub.2 Scrubbing With Sodium Sorbent Compounds," presented at Second EPRI Conference on Fabric Filter Technology for Coal-Fired Power Plants held in Denver on Mar. 22-24, 1983.
Most research to date on dry injection FGD appears to have centered on fundamental investigations of sorbent types and injection rate and FGD temperature conditions for their impact on SO.sub.2 removal. As a consequence, few references describe dry injection FGD procedures and sorbents in sufficient detail to develop a process suitable for commercial use. No references have been found that provide methods for improving overall sorbent efficiency in a full-scale dry-injection FGD process.
Such improvements are reported for other FGD processes (not dry injection) that have undergone more substantial commercial development. A spray-drying process described in U.S. Pat. No. 4,324,770 issued to Bakke obtains an improvement in SO.sub.2 -removal efficiency and alkali (lime) reagent utilization by recycle of the spray-dried powder into the spray dryer. Drawbacks to this process are that large amounts of recycle are called for and, like other spray-drying FGD procedures, that large amounts of energy are consumed in the evaporation of water in the spray drying operation and a dependable water source is required.
The present invention, however, concerns a dry injection FGD process and utilizes a novel recycle procedure that unexpectedly increases the sodium utilization of soda-type dry injection sorbents, without compromising the SO.sub.2 removal efficiency desired for the process.